Molecular biological and histomorphometrical study on bone remodeling and root resorption during orthodontic tooth movement.

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In the present study, the gene expression for osteopontin (OPN) and osteocalcin (OSC) in the alveolar bone was investigated by in situ hybridization histochemistry with non-radioisotopic digoxigenin 11-UTP labeled cRNA prove for OPN and OSC in rat maxillary dent-alveolar unit containing molars and intact PDL following a single loading to produce a well synchronized wave of complete remodeling. From these experiments we found osteocytes respond to mechanical loading in vivo with expression of mRNA for OPN but not for OSC,to associate with subsequent bone resorption. On day 0 an intense positive signal of mRNA for OSC but not OPN was demonstrable at osteoblasts on the distal surface of the interradicular septum of buccal roots of the maxillary first molar of 7-week-old rat. After insertion of an elastic band between the first and second molars the PDL facing the distal surface of the interradicular septum was compressed and the periodontal space on day 1 became narrower than that on day … More 0. At the time the positive signal of OSC mRNA disappeared from cells covering the distal surface of interradicular septum under the compressive strain environment. On the contray, a positive signal of mRNA for OPN was intensely expressed at oseocytes within the matrix in the distal half of the interradicular septum but not in the mesial half, and around the apex of buccomesial root, where the change of consequent compressive strains following a tipping movement of the first molar must occur. Thses osteocytes had large and roudish nuclei, and their osteocyte lacunae became enlarged compared with those on day 0. On day 3, as the compression of PDL continued, lining cells disappeared from the bone surface facing PDL and bone resorption on the distal region of the interradicular septum actively occured. The undermining resorption areas of alveolar bone were prominently eveident, and there were many osteoclasts making Howship's lacunae and a plenty of mononuclear cells around resorbing bone surfaces, where bone matrix containing OPN mRNA expressing osteocytes had been present on day 1. A positive signal of OPN mRNA expressed intensely in osteocytes, multinucleated cells adjacent to Howship's lacunae, designated as osteoclasts, and mononuclear cells on day 3. On the contrary, gene expression for OSC could not be observalbe at these areas.These results suggest that at the beginning of a remodeling cycle stimulated by mechanical force, the OPN in osteocytes promotes osteoclast attachment and activation (stage 1), leading to the resorption phase (stage 2) and that osteocytes play an important role for mechanosensor. Less